JPH075313B2 - Method for producing oxide superconducting thin film - Google Patents

Description

The present invention relates to a method for producing an oxide superconducting thin film. In particular, higher Tc characteristics than Ba ₂ YCu ₃ O _7- δ system are expected
The present invention relates to a method for manufacturing a thin film product in which a Bi-Sr-Ca-Cu-O-based oxide superconducting substance is thinned and its superconductivity is used for devices, wirings and the like.

[Prior Art] Among oxide superconducting materials, it has been confirmed that Bi-based oxide superconducting materials have a phase with a Tc characteristic of 100 K or more. Thinning can be considered by various methods such as sputtering, vapor deposition, and CVD. However, the composition that gives high Tc characteristics, in particular,
The composition ratio of alkaline earth is not determined, and there are multiple superconducting and non-superconducting phases that are thought to differ in composition and crystal structure (c-axis length). It is very difficult to obtain a superconducting phase alone, especially a phase having high Tc characteristics.

Further, it is possible to improve superconducting properties (Tc, Jc, etc.) by adding a different element like Pb addition in Bi system, but in this case also, the optimum addition amount is
It is obtained empirically, not by control adjustment.

When an oxide superconducting material is applied to a thin film, it is very difficult to obtain a superconducting thin film having high superconducting properties because of the same difficulties as those of the sintered body. In addition, when a superconducting substance is formed into a thin film, in order to improve its superconducting property, addition of a different element is being studied as in the case of the sintered body.

That is, when it is attempted to add a different element component to such an oxide superconducting thin film at the thin film forming stage, in particular, K, Pb,
With Tl, which has a high vapor pressure, a compositional deviation often occurs during thin film formation between the sputtering target, vapor deposition source, and other source materials, and the content of the starting material changes over time. However, composition deviation between lots occurs,
It is very difficult to control and adjust the amount of different elements added in the thin film formation stage.

In addition, although it is possible to control the thin film composition by methods used in thin film formation methods such as MBE (molecular beam evaporation method) and CVD (chemical vapor deposition method), it is possible to apply it to oxide superconducting materials. In terms of raw materials, atmosphere control, substrate temperature control, etc.
Many problems still remain.

Further, after the thin film is formed, crystal growth is performed, and during the process of heat treatment for imparting superconducting properties, the additional element in the thin film is further evaporated, and the final addition amount of the different element is
It was impossible to control at the stage of thin film formation.

In addition, it is also considered to artificially form a layered structure possessed by an oxide superconducting material by using a process that is completely different from the sintering of thin film production. This is Y system, Bi system, Tl
It is effective for the oxide superconducting thin film having a long synchronous layered structure such as a superconducting material. Even when artificially designing and manufacturing this laminated film, there are many unclear points at which position in the laminated film, that is, which layer of the laminated film should be added, and the most effective addition at the stage of thin film formation. It is difficult to set the amount or addition position.

[Problems to be Solved by the Invention] In the present invention, therefore, in order to solve the above-mentioned technical problems, the addition of a different element is performed during the heat treatment after the formation of the laminated thin film, and the addition amount thereof is increased. It is an object of the present invention to provide a method for producing an oxide superconducting thin film, which can reliably control and adjust the above.
Another object of the present invention is to provide a method capable of producing a superconducting film having improved superconducting properties and Tc properties.
Further, the present invention aims to provide a method for producing a superconducting thin film, which allows crystal growth of a superconducting thin film to which a different element such as Pb is added in a vapor phase during heat treatment after a laminated thin film is formed. And

[Means for Solving Problems] The present invention provides a Bi ₂ (Sr _1-X Ca _X ) _{N + 1} Cu _N O _Y system (0 <X0.6, Y
= 2N + 4, N is an integer of 2 to 4) In the method for producing an oxide superconducting thin film, when an oxide raw material containing Bi and Sr is used in a direction perpendicular to the substrate surface, (Bi, Sr) The first layer and the second layer of (Ca, Cu) using an oxide raw material containing Ca and Cu are alternately laminated, and the obtained laminated thin film has the respective first and second layers. (Bi, Sr) and (Ca, Cu) are diffused and mixed into each other to form a laminated thin film having a layered structure, and the alternating repetition cycle is several Å to tens of Å. The obtained average composition is that of Bi ₂ (Sr _1-X Ca _X ) _{N + 1} Cu _N O _Y described above, and then this laminated thin film is used in an atmosphere with a high vapor pressure of Pb to be added. Then, a method for producing an oxide superconducting thin film in which Pb is added to the formed laminated thin film by heat treatment is provided. Then, Pb added to the formed laminated thin film by utilizing the vapor phase state, Pb itself or a compound containing Pb is one that has a vapor pressure that evaporates sufficiently at the heat treatment temperature of the thin film. The pressure can cause Pb to diffuse into the laminated thin film. Further, PbO or metallic Pb can be used as an addition source of Pb added. Furthermore, the superconducting property of the laminated thin film can be improved by adding the Pb to be added into the laminated thin film in a vapor phase state.

The method for producing the oxide superconducting thin film used in the present invention is as follows.
ZrO ₂ , SrTiO ₃ , Al ₂ O ₃ (sapphire), BaTiO ₃ or MgO
On the surface of oxide substrate such as sputtering, vacuum deposition,
A PVD (physical vapor deposition) method such as ion plating or cluster ion beam method is used to form a stack of thin films.

That is, in the production of the Bi-based oxide superconducting thin film, the Bi-based oxide superconducting crystal has (Bi, Sr, O) and (Ca, Cu,
The layered structure of (O) can be artificially produced by a sputtering method, and the subsequent heat treatment, for example, 700 to 900 ° C., can easily generate a long period (30 to 40 Å) crystal having high Tc characteristics. The present invention has been accomplished based on the findings.

That is, a layer containing (Bi, Sr) as a main component [a first layer on which an oxide containing Bi and Sr was deposited and formed, using BiSr oxide and CaCu oxide, respectively, as a sputtering target;
Abbreviated as (Bi, Sr)] and a layer containing (Ca, Cu) as a main component [a second layer on which an oxide containing Ca and Cu is deposited and formed;
(Ca, Cu) will be alternately stacked. As a result, the laminated film has concentration modulation of (Bi, Sr) and (Ca, Cu) in the direction perpendicular to the substrate. The average composition of the laminated film should be close to the desired Bi-Sr-Ca-Cu-O-based superconductor composition.
The composition of each layer is adjusted. For example, when an oxide target of Ca _1.0 Cu _0.5 composition and an oxide target of Bi _1.6 Sr _1.0 composition are used, the cation composition of each layer actually obtained is Bi _1.0 S
r _2.0 Ca _5.0 Cu _6.6 and Bi _2.0 Sr _2.0 Ca _1.6 Cu _1.4 are obtained (corresponding to Example 1). As a result, the average composition of the laminated film actually obtained is Bi _2.4 Sr _2.0 Ca _1.8 Cu _2.8 , and Bi ₂ Sr ₂ Ca ₂ Cu ₃ has a desired composition.

Then, a thin film in a laminated state in which the cation concentration of (Bi, Sr) or (Ca, Cu) is alternately increased or decreased in each layer is formed. Specifically, the above specific (Bi, Sr)
In this state, thin film layers having a concentration gradient of (Ca, Cu) are alternately stacked. Ideally, (Bi, Sr) and (Ca, Cu) are completely separated, but due to the effect of interface diffusion during stacking, the (Bi, Sr) layer actually has ( Ca, Cu)
However, since (Bi, Sr) is slightly diffused and mixed into the (Ca, Cu, O) layer, a concentration gradient occurs in each thin film layer obtained.

Further, at the time of heat treatment, a different element such as Pb is added to the laminated thin film via the gas phase, that is, PbO or Pb vapor using PbO or Pb metal as an addition source, thereby promoting the crystal growth of the superconducting phase. It is intended to improve superconducting properties. Therefore, the method for producing a superconducting thin film of the present invention can be applied to the production of other oxide superconducting thin films.

According to the present invention, the foreign element Pb added in the superconducting thin film
Was added to the thin film through a gas phase, that is, PbO vapor or Pb vapor during heat treatment of the thin film, using PbO or Pb metal as an addition source. Therefore, thin film heat treatment temperature (≤900 ℃)
It is also possible to use an element having a sufficiently high vapor pressure, such as Tl or K, and it is possible to add it to the thin film by the same method. Further, it is possible to add a plurality of different elements.

That is, these elements (Pb, Tl, K, etc.) serve to improve the superconducting properties by partially substituting mainly Bi sites of a desired crystal to be formed. These elements are volatile,
While quantitatively controlling the composition by the sputtering method,
It is difficult to form a thin film, and according to the present invention, the purpose is to add it from the gas phase into the film during heat treatment.

In the present invention, the laminated film is (Bi, Sr, O) (Ca, Cu,
Each layer of O) has a thickness of 5 to 15 Å and has a period of about 20 Å.
In order to obtain the vapor phase addition effect of the different elements after forming the thin film, the cycle and combination of the laminated films may be other than these. PVD other than sputtering method
Method or CVD method can be used.

That is, the additive element such as Pb has a vapor pressure that evaporates sufficiently at the processing temperature of the thin film, and by utilizing this vapor pressure, it is added to the laminated thin film in the vapor phase and diffused. Laminated thin films to which dissimilar elements such as Pb are added are
In each of the (Bi, Sr) and (Ca, Cu) layers, a concentration gradient due to diffusion occurs, and the repeating period (stacking period) is as described above, and as a result, the average composition obtained is as described above. Become.

Then, regarding the layer thicknesses of the first layer and the second layer which are alternately laminated and deposited, for example, [10Å / 15Å] represents the ratio of the thicknesses of the (BiSrO) unit and the (CaCuO) unit, and, for example, In Example 1, [10Å / 10Å], and Bi ₂ Sr ₂ Ca ₂ Cu
The minimum repeating unit of ₃ O ₁₀ is 37Å (c-axis length) / 2 ≈ 18Å
Assuming that, as a small virtual layer thickness in this, Bi ₂ S
Approximately 10 Å of thickness is allocated to roughly correspond to r ₂ O ₅ unit and Ca ₂ Cu ₃ O ₅ unit. In this way, a structure that is as close as possible to the desired structure (however, in the amorphous state) is created in advance, and low energy (low heat treatment temperature) is used.
So I decided to crystallize it. This is also effective in growing a desired crystal by adding Pb from the gas phase during the heat treatment according to the present invention.

The different element used in the present invention is the heat treatment temperature (70
An element having a sufficiently high vapor pressure at 0 to 900 ° C. (for example, Tl, K, etc.) can be added to the laminated thin film by the same method. Furthermore, it is possible to add a plurality of elements.

After forming the laminated thin film on the single crystal substrate, when performing the heat treatment, for example, after forming the laminated thin film by sputtering at room temperature and then performing the heat treatment, the heat treatment is performed in the vapor atmosphere of the different elements such as PbO. Is added.

The gas phase addition of such a different element (Pb) is carried out in a ceramic (Al ₂ O ₃ ) incomplete sealed container with the laminated thin film sample and P
Simply put bO powder or pellets or Pb grains together, heat-treat them simultaneously in air or oxygen at a temperature of 700-900 ° C, and use vaporization of PbO to add them to the formed thin film in the vapor phase. It is done by the method.

[Operation] When the heterogeneous element PbO according to the present invention is added in the vapor phase, the addition amount thereof can be controlled and adjusted by the amount of the evaporation source placed in the furnace for the heat treatment, the heat treatment temperature, and the time.

Therefore, the method for producing a superconducting thin film produced according to the present invention involves heat treatment and can be applied to the production of a thin film that requires a large area, and can also be applied to the production of wiring, antennas, coils, etc. is there.

Next, the method for producing the oxide superconducting laminated thin film of the present invention will be specifically described by way of Examples, but the present invention is not limited thereto.

[Example 1] A laminated thin film was formed on a single crystal surface (100) of an MgO substrate by a high frequency magnetron sputtering method. The composition of each layer of the laminated film was Bi _2.6 Sr _2.0 Ca _1.6 Cu _1.4 O _X , film thickness about 10 Å and Bi _1.0 Sr _2.0 Ca _5.0 Cu _6.6 O _Y , film thickness about 10 Å, and the average composition was Bi _2.0 Sr _2.0. Ca _2.0 Cu _3.0 O _z , so stacking period 20
Have Å. Therefore, the laminated thin film has a concentration distribution of (Bi, Sr) and (Ca, Cu) every 10Å in the direction perpendicular to the substrate surface, and the total film thickness was 2000Å.

For comparison, a Bi—Sr—Ca—Cu—O-based homogeneous thin film was prepared on the MgO (100) single crystal surface by the high frequency magnetron sputtering method. The produced homogeneous film has a film thickness of 2000Å
And had the composition Ba _2.0 Sr _1.9 Ca _1.9 Cu _2.6 O _Z and was in an amorphous state.

That is, when comparing the laminated film according to the present invention with the homogeneous film having no laminated structure for the above comparison, the formation of desired Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ crystals was observed in the laminated film according to the present invention. On the other hand, it was not observed in the homogeneous film.

The thin film formation conditions are, without regard to substrate heating, an atmosphere gas of Ar: O ₂ = 9: 1, in both laminated and homogenous films, and a high frequency output of 100 W under a pressure of 5 mTorr.

This heat treatment is performed on an alumina boat (internal volume of about 10 x 10 x 60
mm ³ ), the laminated thin film and the homogeneous thin film sample were placed, and 0.05 g of PbO powder was further placed in the same boat, the lid was covered with an alumina boat, and heat treatment was performed in air in a muffle furnace. The heat treatment temperature was 880 ° C. for 8 hours. The temperature was raised rapidly by putting the sample in a furnace maintained at 880 ° C., cooling was allowed to cool in the furnace, and the sample was taken out after cooling to a temperature of 200 ° C. or lower.

The PbO powder in the alumina boat was melted by the heat treatment.

The heat-treated homogeneous film sample was subjected to Auger electron spectroscopy (AES), and the results are shown in FIG.
The sample at this time has a film thickness of 1000 Å, the kinetic energy (eV) of the detected electron is plotted on the horizontal axis, and its differential intensity (E × dN / dE) is plotted on the vertical axis. Although a slight amount, an Auger peak indicating the presence of Pb was observed, indicating that Pb was added to the formed thin film.

In addition, the X-ray diffraction lines of the laminated and homogeneous thin film samples after the heat treatment were measured. The results are shown in FIG. It can be seen that both the laminated film and the homogeneous film grow with c-axis orientation. However, it can be seen that the laminated film has a higher diffraction peak intensity and better crystallinity than the homogeneous film.

Furthermore, in the laminated film, it is considered that Tc = 110K phase. A superconducting phase with a c-axis length of 37Å is growing.

Furthermore, FIG. 3 shows the temperature dependence of the electrical resistivity of the thin film sample after the heat treatment. Compared to homogeneous film, laminated film has Tc, ze
ro was several degrees higher. In addition, the decrease in the resistivity at 110 K due to the crystal phase having a c-axis length of 37 Å is also larger in the laminated film.

The graph shown in FIG. 3 is obtained by measuring the electric resistance of a film having a film thickness of 2000 Å which was heat-treated in air at 880 ° C. for 8 hours. The processing temperature is plotted on the abscissa and the electrical resistivity is plotted on the ordinate.

That is, it was revealed that the Pb vapor phase addition treatment promotes the crystal growth of the superconducting phase, and the characteristics such as Tc, onset and room temperature resistance are improved in the laminated film as compared with the homogeneous film.

[Example 2] Under the same thin film forming conditions as in Example 1, a laminated thin film having the same thin film composition and film thickness was heat-treated in air at 880 ° C in PbO vapor for 8 hours. At this time, the amount of PbO powder placed in the alumina boat was set to 1 g, which was 20 times the amount in Example 1.

The result of measuring the X-ray diffraction (CuKα) of the thin film sample obtained by the above heat treatment is shown in FIG.

Increasing the amount of PbO placed in the boat during heat treatment promotes crystal growth of the superconducting phase in the thin film,
It can be seen that the diffraction peak with a c-axis length of 37Å ( mark, 2θ = 4.8 °) became sharp, and the Tc = 110K phase crystal grew.

In the heat-treated laminated thin film, the temperature at which the superconducting transition started was about 110K.

[Example 3] As in Example 1, a superconducting thin film was formed, and a laminated film having a film thickness of 2000Å was formed on a MgO (100) plane substrate.

The laminated thin films produced in this manner are (Bi, Sr, O) and (C
The density modulation width of a, Cu, O) is 10Å / 10Å (Example 1)
Same as above) and 10Å / 15Å two kinds of laminated thin films were prepared.
That is, the average composition of the latter laminated film is Ca, Cu compared to the former.
There are many. The heat treatment was performed in the same manner as in Example 1 (that is, PbO).
Powder (0.05 g) at 820 ° C. for 20 hours.

The superconducting property of the laminated thin film heat-treated in this way is measured,
The results are shown in Table 1. Table 1 Sample 10Å / 10Å 10Å / 15Å Tc, onset 108K 125K Tc, mid 81K 81K Tc, zero - at 75K room temperature resistance 442mΩ · cm low heat treatment temperature of 2.6mΩ · cm 820 ℃, the 10Å / 10Å, Tc, ze
ro was not obtained above the liquid helium temperature, but by changing the cycle of the laminated film, for example, 10Å / 15Å, T
A high value of c, zero = 75K was obtained. Also, Tc, onse
The t was as high as 125K.

By appropriately changing and adjusting the cycle of the laminated film in this manner, the superconducting property can be improved in addition to the effect of the heat treatment in the PbO vapor.

Tc and onset = 125K were obtained at a temperature of 820 ° C, which is lower than the usual heat treatment temperature by several tens of ° C.

[Example 4] Under the same conditions as in Example 1, a laminated film having a period of 10Å / 10Å was formed to a thickness of 2000Å. This thin film was heat-treated in the same manner as in Example 1 in a PbO vapor atmosphere at 850 ° C. for 5 hours or 10 hours. The thin-film characteristics of this heat-treated superconducting thin film were similarly measured, and the results are shown in Table 2. Table 2 Heat treatment time 5 hours 10 hours Tc, onset 115K 114K Tc, zero-66K Room temperature resistance 9.0mΩ ・ cm 1.6mΩ ・ cm Next, XRD analysis was carried out on these samples, and as a result, any sample c The phase with an axial length of 30Å is c
It had grown axially.

Combined with the results of Examples 1 and 2, in order to give superconductivity to a laminated film having a period of 10Å / 10Å by heat treatment in PbO vapor, a simple method of treating at a high temperature of about 880 ° C. It is possible. Further, in order to give superconductivity at a lower temperature, a long heat treatment such as 850 ° C. for 10 hours and 820 ° C. for 20 hours or more is required.

[Advantages of the Invention] The method for producing a superconducting thin film by heat-treating a formed thin film according to the present invention brings the following remarkable technical effects.

First, it was possible to provide a method for producing a superconducting thin film capable of significantly improving superconducting properties.

Secondly, the heat treatment after forming the laminated thin film on the substrate
The present invention provides a method for producing a superconducting thin film which is most effective and at the same time has excellent superconducting properties.

Thirdly, it is possible to provide a manufacturing method capable of easily manufacturing a highly stable and highly reliable oxide superconducting thin film, which was not possible in the past.

[Brief description of drawings]

FIG. 1 is an Auger electron spectroscopy (AES) diagram of a superconducting thin film prepared according to the present invention. FIG. 2 is an X-ray diffraction diagram of the laminated thin film and the homogeneous thin film produced by the present invention after heat treatment in PbO vapor. FIG. 3 is a graph showing a change in electric resistivity with respect to temperature of the superconducting thin film manufactured by the present invention. FIG. 4 is an X-ray diffraction diagram of the superconducting thin film produced by the present invention after heat treatment in PbO vapor.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H01B 13/00 ZAA 7244-5G 565 D 7244-5G H01L 39/24 ZAA B 9276-4M

Claims (4)

[Claims] 1. Preparation of Bi ₂ (Sr _1-X Ca _X ) _{N + 1} Cu _N O _Y system (0 <X <0.6, Y = 2N + 4, N is an integer of 2 to 4) oxide superconducting thin film. In the method, a first layer of (Bi, Sr) using an oxide raw material containing Bi and Sr and a (Ca , Cu) second layers are alternately laminated, and the formed laminated thin film has a concentration gradient in which (Bi, Sr) and (Ca, Cu) are diffused and mixed in each of the first and second layers. Is a laminated thin film having a layered structure, and the cycle of alternately repeating lamination is several Å to several tens of Å, and the obtained average composition is Bi ₂ (Sr _1-X Ca _X ) _{N +1} Cu _N O _Y. Next, this laminated thin film is heat-treated in an atmosphere with a high vapor pressure of Pb to be added, and Pb is added to the formed laminated thin film. Method of forming oxide superconducting thin film . 2. The Pb added to the formed laminated thin film by utilizing the vapor phase state is such that Pb itself or a compound containing Pb has a vapor pressure at which the thin film is vaporized sufficiently at the heat treatment temperature of the thin film. The method for producing an oxide superconducting thin film according to claim 1, wherein Pb is diffused in the laminated thin film by the vapor pressure. 3. The method for producing an oxide superconducting thin film according to claim 1, wherein PbO or metal Pb is used as an addition source of Pb to be added. 4. The superconducting property of the laminated thin film is improved by adding Pb to be added to the laminated thin film in a vapor phase state. Method for producing oxide superconducting thin film of.